The present invention relates to dimmers for use in connection with solid state neon tube power supplies, in particular, to dimmers for high frequency power supplies operating at frequencies generally above 10 Khz.
Most conventional high frequency neon power supplies operate at a fixed current output determined by power supply design and the length of the neon tube or tubes connected thereto. Such supplies are, in short, operated at a single output level corresponding to full or maximum light intensity.
While fixed full-output neon supplies are satisfactory for most applications--usually for outdoor or window advertisement applications --there is growing demand for lower or variable intensity neon signage principally for indoor applications where normal high intensity illumination does not comport with the subdued and darkened atmosphere associated with many food and beverage establishments--common users of neon signage. The present invention, therefore, pertains to a dimmer arrangement for high frequency neon power supplies that permits the continuous adjustment of light output from full intensity down to a low light output level of, for example, about 5-10% thereof.
In certain instances a conventional SCR or triac-type `conduction angle` or pulse width modulation (PWM) lamp dimmer may be employed to vary the light intensity, particularly where the neon sign is powered from a standard 60 Hz power transformer supply. And it might reasonably be assumed that the PWM dimming scheme could be extrapolated to high frequency neon power supplies as this is the principle upon which many high frequency switching power supplies operate.
Several problems, however, have been encountered when applying PWM dimmer technology to high frequency neon power supplies. These include the non-uniform illumination of the neon tube and the lowering of the output voltage below that necessary to assure neon gas excitation--both phenomena occurring at lower illumination intensities.
As presently understood, the reason for the first of these limitations is related to the distributed tube capacitance which may be as high as 50 picofarads or more. This capacitance progressively shunts tube current to ground along the length of the tube, that is, as viewed by moving from the respective tube ends toward the center. As the voltage across the tube is substantially independent of tube current (actually, the negative resistance characteristic of the neon tube results in a slightly increasing tube voltage with lowering tube currents), this capacitive leakage current is also substantially independent of tube illumination or dimmer setting. For a 20 KHz neon supply and typical neon tube, this current is approximately 12 milliamperes.
By comparison, a neon tube current of about 25 milliamperes is typical for normal (full) neon tube illumination. As these two current components (i.e. tube leakage and tube illumination currents) are in quadrature, a total supply current of under 28 ma results. Thus, it will be appreciated that the leakage current causes only a negligible reduction in neon tube current for normal tube illumination intensities and consequently this gradual current reduction along and toward tube center produces a correspondingly trivial reduction in light intensity.
This is not the case for lower tube illumination intensities, however. Take, for example, a tube dimming of 80%, that is where the desired tube current is 20% of full tube intensity current of 5 ma. For this configuration (i.e. quadrature leakage and illumination currents of 12 and 5 ma, respectively) the total supply current is computed to be 13 ma. It should be observed, however, that the full 13 ma supply current enters the neon tube ends as all of the capacitive leakage and tube neon currents flow through these points. Thus, the tube ends are illuminated not by a mere 5, but a 13, milliampere current.
The current through the center section of the tube (which is at "ground" potential by reason of the balanced nature of the supply output), however, is the previously specified 5 ma --the 12 ma quadrature leakage current having been fully shunted to ground. The tube is therefore illuminated to a 5 ma intensity in the center, but gradually increases to 13 ma at the ends. This differential produces a clearly visible and objectionable illumination non-uniformity that only gets worse as greater dimming levels are selected.
The second limitation of PWM neon supplies relates to the intrinsic low pass filter characteristic of the power supply and neon load. This filter characteristic--which has a cut-off frequency generally of twice the supply operating frequency--is created by the series "leakage" inductance of the high voltage transformer working against the secondary inductance and capacitance and the previously mentioned tube leakage capacitance.
The oscillator output waveform, for ordinary `full output` operation, is of generally symmetrical form having substantial energy at the fundamental or operating frequency. Thus, the above-mentioned low pass characteristic is of minimal consequence for ordinary operation. However, as the pulse widths are narrowed by the PWM circuitry (as occurs upon dimming with this conventional approach), the relative fundamental energy content of the resulting output waveform drops dramatically. And by reason of the above-discussed low pass filter characteristic, the remaining high frequency harmonic energy is not coupled to the neon tube and therefore does not significantly contribute to the available excitation voltage thereof. As dimming is increased (i.e. as the pulse widths narrow) the neon tube excitation voltage may drop below the requisite ionization potential thereby resulting in erratic and unreliable tube operation, specifically, the failure of the tube to illuminate or an oscillatory-type flickering or blinking thereof.
The present invention pertains to various arrangements to avoid the above-noted dimmer problems and to improvements in ground fault interruption (GFI) circuitry to permit the proper operation thereof. One approach contemplated by the present invention employs a pulse frequency modulation technique in which the repetition rate of so-called "full brilliance" pulses, (i.e. pulses of an amplitude which, if continuous, would effect full tube illumination and, further, of a period that corresponds to a generally optimal power supply operating frequency, e.g. 20 KHz), is selectively adjusted to cause corresponding variations in the average tube current, in turn, to the overall brightness of the luminous tube. It will be appreciated that while the average current may be low, the actual current through the tube during any given pulse corresponds to the full illumination current, e.g. about 25 ma, and therefore that the above-described problems of unequal tube dimming and tube non-excitation are eliminated.
In some instances it has been found that the above dimming arrangement produces objectionable noise in the form of an audible acoustic squeal as dimming levels are increased (i.e. illumination intensities are lowered). As greater "dimming" is selected, the repetition rate of the high frequency (e.g. 25 KHz) pulses is correspondingly lowered and may fall well within the audible range, for example, 500 Hz-10 KHz. More specifically, magneto-restriction and Lenz Law forces effectively serve to create an acoustic transducer which is, in turn, driven by the lowered audio frequency pulses present during reduced intensity power supply operation.
Therefore another embodiment of the present invention is proposed in which groups of high frequency pulses (these pulses, again, being in the order of about 20 KHz) are applied to the luminous tube at a generally low frequency rate. A rate above the so-called "flicker rate" at which the human eye perceives visible flicker, for example about 100 Hz, is preferred. At such a low frequency, the problems of magneto-restriction and Lenz Law induced acoustic noise are greatly reduced and, to the extent that any such noise remains, the low frequency thereof renders the resulting noise less objectionable.
The selective adjustment of dimming is preferably controlled by varying the duration of each of the "pulse groups", that is, the number of high frequency pulse cycles contained therein. In this manner a full range of dimming can be achieved while maintaining the fundamental high frequency excitation and low frequency repetition rates. It will be understood, however, that a combination of the above-described pulse frequency and pulse group modulation techniques may be employed whereby both the repetition rate and number of pulses found in each pulse group may be selectively varied to achieve dimming operation.
Another feature of the present invention relates to ground fault interruption or GFI circuitry. Ground fault interruption--i.e. the switching "off" of the power supply upon the detection of an unexpected and potentially lethal ground path current--has long been the practice and, often, the requirement of applicable safety codes. Ordinary ground fault circuits, however, have been found to incorrectly indicate a ground fault condition when used in conjunction with the pulse group modulation dimmer of the present invention.
It has been found that the low pulse group repetition rate, with its corresponding long "off" periods between sequential pulse groups, allows the neon gas to deionize. In view of the fact that, first, neon does not instantaneously re-ionize upon the corresponding re-application of high voltage and, second, that the neon ionization wavefront does not propagate from each power supply electrode at the same rate, a short term (about 100 uSec) current flow imbalance occurs which imbalance may, in turn, falsely trigger the GFI circuitry.
The present GFI circuit therefore provides a mechanism for detecting the commencement of a new pulse group and a switch means, in turn, for momentarily inhibiting ground fault operation for a period sufficient to assure that any ground fault signals are real and not, as above-described, inducted by idiosyncracies of the neon gas, itself. The total duration of such inhibiting, being in the order of a few hundred microseconds, is not sufficiently long to pose a health hazard.